ABSTRACT Breast cancer (BC) is the most commonly diagnosed cancer and cause of cancer mortality in Asian/Pacific Islander (API) and Hispanic women, the two most rapidly growing minority ethnic groups in the US due to immigration. Once diagnosed, immigrant API women have similar or worse, and immigrant Hispanic women have better, BC survival than ethnically similar, US-born women, the latter despite later stage at diagnosis. Investigators have frequently speculated that social networks play an important role in BC outcomes among immigrant populations, through beneficial health behaviors that are maintained due to living in neighborhoods characterized as ?ethnic or immigrant enclaves? or through adverse behaviors related to reduced availability of resources. At the individual level, in largely non-Hispanic white populations, women with large supportive social networks have been shown to have better health behaviors and BC survival. However, no research has explored the multilevel (neighborhood and individual) influence of social networks on health behaviors and BC outcomes in diverse API and Hispanic immigrant women. Therefore, in a large (N=6,184), pooled northern California cohort of 683 Hispanic (41% immigrant), 851 API (59% immigrant), and 4,650 (8% immigrant) NHW women from the Pathways, the Life After Cancer Epidemiology, and the Asian CHI studies, diagnosed with stage I-IV BC, we propose to: 1) Evaluate associations between immigrant status and social networks at the neighborhood level (i.e., immigrant and ethnic enclaves and sociocultural institutions) and at the individual level (i.e., self-reported size, diversity, quality, and type of social ties), overall and by race/ethnicity; 2) Evaluate associations between immigrant status and multilevel social network characteristics with behavioral factors known to influence BC survival (Western diet, soy and dairy consumption, MET-hours of physical activity, amount and frequency of alcohol intake, current and pack years of smoking, obesity, and waist size), overall and by race/ethnicity; and 3) Evaluate associations between immigrant status, multilevel social network characteristics, and BC outcomes including recurrence, BC-specific mortality, and total mortality, overall and by race/ethnicity, evaluating the degree to which social network characteristics and health behaviors mediate associations. We will use meta-analytic techniques to evaluate study heterogeneity; linear, logistic, and generalized linear mixed effects (multilevel) models to analyze associations in Aims 1 and 2; and multi-level Cox proportional hazards regression to evaluate associations in Aim 3. The unique convergence of detailed data on immigrant status, race/ethnicity, multilevel social networks, highly detailed behavioral measures, and BC outcomes, provides the first opportunity to examine the multilevel influence of social networks on health behaviors and BC outcomes in diverse immigrant women with BC, adjusted for an extensive set of covariates. This work is central to identifying factors underlying BC disparities in ethnically diverse immigrant women, particularly those factors underlying favorable BC outcomes despite socioeconomic disadvantage.